Frequency modulation



y 1,944. D. M. KALTENBACHER 2,353,162

FREQUENCY MODULATION Filed May 29, 1942 FIG. 2.

Zhwentor I Domin icM Kaltenbacher u;

CM L Qttomeg Patented July 11, 1944 FREQUENCY MODULATION Dominic M. Kaltenbacher, Bloomington, Ind., assignor to Radio Corporation of America, a corporation of Delaware Application May 29, 1942, Serial No. 445,008 8 Claims. (Cl. 119-1715) While various methods for sound detection have been utilized in the past, this invention provides a simple and improved means for frequency modulating an oscillatory circuit in response to sound waves impinging upon a conventional acoustic diaphragm. The diaphragm is arranged to drive a magnetic core and a capacitor electrode to providesimultaneous variation of an inductor and a capacitor in the oscillation circuit of a thermionic tube oscillator. 7 Any conventional type of oscillator circuit may be used and provision may be included for adjusting the carrier frequency thereof. Preferably the output of the oscillator should be coupled through a suitable high frequency amplifier designed to amplify efficiently the frequency band required. The output of the amplifier may be coupled to a transmission line or to an antenna for radiating frequency modulated high frequency waves. In either instance, a suitable receiver for frequency-modulated high frequency signals should be provided for reproducing the frequency modulated signals as sound waves. A plurality of such sound pickup devices may be the sound waves.

The invention will be described by reference to the drawing in which Figure 1 is a schematic circuit diagram of one embodiment of the complete system; and Figure 2 is a schematic diagram of a modification thereof.

Referring to the drawing, sound waves S drive the diaphragm l2 in the direction indicated by the arrow. A thermionic tube oscillator l is supplied with cathode, control electrode and anode operating voltages in any conventional manner. The control electrode circuit includes a resonant circuit comprising the inductor 2 and the capacitor 3 connected in parallel and designed to resonate at the desired carrier frequency. The tube anode circuit includes a parallel resonant circuit comprising the inductor 4 connected in parallel with the capacitor 5. A variable capacitor 8 may be connected in parallel with the first capacitor 5 to adjust the anode circuit to resonance with the control electrode employed, and operated on the same or different carrier frequencies, for reception by any suitable frequency modulation receiver equipment.

Among the objects of the invention are to provide a new and improved means for converting sound waves into corresponding simultaneous variations of inductive and capacitive-reactance. Another object of the, invention is to provide a new and improved means for detecting sound waves by means of a suitable acoustic diaphragm, varying simultaneously the inductive and capacitive reactance of a tuned oscillatory circuit in response to the sound waves, and transmitting a frequency modulated signal in which the modulation is derived from the variable reactance device. A still further object of the invention is to provide a newand improved means for detecting sound waves by a suitable acoustic diaphragm, simultaneously, and in the same sense, varying thereby the inductive and capacitive reactance of a tuned oscillatory circuit, and transmitting the resulting frequency modulated circuit, whereby the complete oscillator circuit corresponds to the conventional tuned-grid, tuned-plate oscillator well known in the art. A movable magnetic core 6, which maybe of the powdered iron type, and a movable capacitor electrode 1 are both rigidly connected to the diaphragm l2 and arranged to oscillate coaxially with the axes of the inductor 4 and the capacitor 5. Thus, movement of the diaphragm l2 in response to detected sound waves provides simultaneous variation of the inductive and capacitive reactance of the tuned anode circuit. The simultaneous variation of both inductive and capacitive reactance provides a highly efiicient method of frequency modulating an oscillator circuit. Oscillations derived from the oscillator circuit are coupled through the capacitor 9 to a suitable high frequency band pass amplifier l0, and are radiated as frequency modulated radio waves by means of the antenna H. At any desired remote point the frequency modulated signals are received 0n the antenna 12 of the frequency modulation receiver I3, which may be of any type well known in the art. The received signals are then reproduced as sound waves by the reproducer ll.

It should be understood that any desired number of sound pickup circuits may be utilized, and that frequency modulated signals therefrom may be radiated at the same or different frequencies. It should also be understood that the signals may be transmitted from the amplifierv Hi to the. receiver H by any suitable high frequency transmission line. If the apparatus is to be utilized for the detection of predetermined audible irequencies, the diaphragm I2 should Preferably be designed to resonate thereto, and may include a suitable acoustic amplifier, not shown herein.

Figure 2 is a modification oi the sound conversion device included in the circuit of Fig. 1. A conical diaphragm l2 drives a magnetic diaphragm 22 in response to sound waves received thereon The reactance of a magnetic circuit, including a U shaped core 23 and the windings 24, is varied by the magnetic diaphragm 22 disposed adjacent the ends of the core 23. Simultaneously the magnetic diaphragm 22 varies the capacitive reactance of a circuit including the magnetic diaphragm 22 and a capacitor electrode 25 disposed adjacent and parallel thereto. The magnetic and capacitive circuits may be electrically connected either in series or parallel, as desired. Preferably, the variations of inductive and capacitive reactance shouldbe of approximately the same magnitude in response to sound waves, to provide a device of maximum sensitivity and efllciency.

I claim as my invention:

1. In apparatus for converting sound waves into timing modulated currents, a multi-electrode thermionic tube, a tuned oscillatory circuit including a variable inductance and a capacity formed by at least two conducting elements between which efiective capacity exists which capacity is variable,the said variable inductance and variable capacity being in said oscillatory circuit and coupled to electrodes of said tube, means for causing oscillatory current to fiow insaid oscillatory circuit, and means responsive to said sound waves for simultaneously varying said variable inductance and said variable capacity formed by said elements to change the tuning oi said oscillatory circuit to modulate the timing of the oscillatory energy flowing in said oscillatory circuit.

2. Apparatus of the type described in claim 1 including load means coupled to said circuit.

3. Apparatus for converting sound waves into frequency modulated current including a thermionic tube oscillator, a tuned oscillatory circuit including an inductor and a capacitor element connected to said tube, and means responsive to said sound waves including a magnetic core and a second capacitor element simultaneously movable with respect to said inductance and said first capacitor element respectively for varying simultaneously said inductance and the capacity formed by said capacitor elements for frequency modulating said oscillator.

4. Apparatus oi the type described in claim 3 including a diaphragm responsive to said sound waves and operating means connecting said diaphragm to said core and said second capacitor element.

5. Apparatus of the type described in claim 3 including load means coupled to said oscillator.

6. Apparatus for converting sound waves into frequency modulated currents including a thermoscillatory ionic tube oscillator, a tuned oscillatory circuit including an inductor and a capacitor element connected to said tube, and means responsive to said sound waves including a magnetic core and a second capacitor element simultaneously movable with respect to and coaxial with said inductance and said first capacitor element respectively for varying simultaneously said inductance and the capacity formed by said capacitor elements for frequency modulating said oscillator.

7. Apparatus or the type described in claim 1 including an adjustable capacitor connected in said oscillatory circuit for adjusting the carrier frequency oi said apparatus.

8. Apparatus oi the type described in claim 3 including an adjustable capacitor connected in said oscillatory circuit for adjusting the carrier frequency of said apparatus.

DOMINIC M. KALTENBACHER. V 

